Micheal A. Davis

Irreconcilable Differences: Fine-Root Life Spans and Soil Carbon Persistence

The residence time of fine-root carbon in soil is one of the least understood aspects of the global carbon cycle, and fine-root dynamics are one of the least understood aspects of plant function. Most recent studies of these belowground dynamics have used one of two methodological strategies. In one approach, based on analysis of carbon isotopes, the persistence of carbon is inferred; in the other, based on direct observations of roots with cameras, the longevity of individual roots is measured. We show that the contribution of fine roots to the global carbon cycle has been overstated because observations of root lifetimes systematically overestimate the turnover of fine-root biomass. On the other hand, isotopic techniques systematically underestimate the turnover of individual roots. These differences, by virtue of the separate processes or pools measured, are irreconcilable.

Nickel defends the South African hyperaccumulator Senecio coronatus (Asteraceae) against Helix aspersa (Mollusca: Pulmonidae).

SummaryThe elevated Ni concentration of Ni hyperaccumulator plants has been proposed to be an effective chemical defence against herbivores. To test this hypothesis, we fed leaves from hyperaccumulator and non-hyperaccumulator populations of South African Senecio coronatus to a generalist herbivore species, the brown garden snail (Helix aspersa). Snails fed hyperaccumulator leaves experienced significantly greater mortality than those fed non-hyperaccumulator leaves and also contained 10-fold greater concentrations of Ni. A choice experiment showed snails preferred non-hyperaccumulator leaves in two of three trials. Snails fed cornmeal diet amended with Ni had significantly reduced mass for diets containing as little as 140 μg Ni g-1, and significantly greater mortality occurred for snails consuming diets containing 830 μg Ni g-1 and greater. Because hyperaccumulator S. coronatus leaves contained far more Ni (12,100 μg Ni g-1) than the toxic threshold shown in the diet experiment, we concluded that the Ni concentration of hyperaccumulator leaves was sufficient to cause the elevated mortality of snails fed those leaves. This research adds another example to the growing literature showing the toxicity of hyperaccumulated Ni to generalist folivores.

Nickel hyperaccumulation by Streptanthus polygaloides protects against the folivore Plutella xylostella (Lepidoptera: Plutellidae)

We determined the effectiveness of Ni as an elemental defence of Streptanthus polygaloides (Brassicaceae) against a crucifer specialist folivore, diamondback moth (DBM), Plutella xylostella. An oviposition experiment used arrays of S. polygaloides grown on Ni-amended (high-Ni) soil interspersed with plants grown on unamended (low-Ni) soil and eggs were allowed to hatch and larvae fed freely among plants in the arrays. We also explored oviposition preference by allowing moths to oviposit on foil sheets coated with high- or low-Ni plant extract. This was followed by an experiment using low-Ni plant extract to which varying amounts of Ni had been added and an experiment using sheets coated with sinigrin (allyl glucosinolate) as an oviposition stimulant. Diamondback moths laid 2.5-fold more eggs on low-Ni plants than on high-Ni plants and larval feeding was greater on low-Ni plants. High-Ni plants grew twice as tall, produced more leaves, and produced almost 3.5-fold more flowers. Low-Ni plants contained more allyl glucosinolate than high-Ni plants and moths preferred to oviposit on foil sheets dipped in low-Ni plant extract. Moths showed no preference when Ni concentration of low-Ni extract was varied and overwhelmingly preferred sinigrin coated sheets. We conclude that Ni hyperaccumulation is an effective elemental defence against this herbivore, increasing plant fitness through a combination of toxicity to DBM larvae and decreased oviposition by adults.

Metal concentrations of insects associated with the South African Ni hyperaccumulator Berkheya coddii (Asteraceae)

Abstract The high levels of some metals in metal hyperaccumulator plants may be transferred to insect associates. We surveyed insects collected from the South African Ni hyperaccumulator Berkheya coddii to document whole‐body metal concentrations (Co, Cr, Cu, Mg, Mn, Ni, Pb, Zn). We also documented the concentrations of these metals in leaves, stems and inflorescences, finding extremely elevated levels of Ni (4 700–16 000 ∞g/g) and high values (5–34 ∞g/g) for Co, Cr, and Pb. Of 26 insect morphotypes collected from B. coddii, seven heteropterans, one coleopteran, and one orthopteran contained relatively high concentrations of Ni (> 500 ∞g/g). The large number of high‐Ni heteropterans adds to discoveries of others (from California USA and New Caledonia) and suggests that members of this insect order may be particularly Ni tolerant. Nymphs of the orthopteran (Stenoscepa) contained 3 500 ∞g Ni/g, the greatest Ni concentration yet reported for an insect. We also found two beetles with elevated levels of Mg (> 2 800 ∞g/g), one beetle with elevated Cu (> 70 ∞g/g) and one heteropteran with an elevated level of Mn (> 200 ∞g/g). Our results show that insects feeding on a Ni hyperaccumulator can mobilize Ni into food webs, although we found no evidence of Ni biomagnification in either herbivore or carnivore insect taxa. We also conclude that some insects associated with hyperaccumulators can contain Ni levels that are high enough to be toxic to vertebrates.

Host-herbivore studies of Stenoscepa sp (Orthoptera : Pyrgomorphidae), a high-Ni herbivore of the South African Ni hyperaccumulator Berkheya coddii (Asteraceae)

Nymphs of Stenoscepa sp. feed on leaves of the Ni hyperaccumulator Berkheya coddii at serpentine sites in Mpumalanga Province, South Africa. These sites contain Ni hyperaccumulators, Ni accumulators, and plants with Ni concentrations in the normal range. We conducted studies to: (i) determine the whole‐body metal concentration of nymphs (including those starved to empty their guts); (ii) compare Stenoscepa sp. nymphs against other grasshoppers in the same habitat for whole‐body metal concentrations; and (iii) compare the suitability of Ni hyperaccumulator and Ni accumulator plants as food sources for Stenoscepa sp. and other grasshoppers. Stenoscepa nymphs had extremely high whole‐body Ni concentrations (3 500 μg Ni/g). This was partly due to food in the gut, as starved insects contained less Ni (950 μg Ni/g). Stenoscepa nymphs survived significantly better than other grasshoppers collected from either a serpentine or a non‐serpentine site when offered high‐Ni plants as food. In a host preference test among four Berkheya species (two Ni hyperaccumulators and two Ni accumulators), Stenoscepa sp. preferred leaves of the Ni hyperaccumulator species. A preference experiment using leaves of three Senecio species (of which one species, Senecio coronatus, was represented by both a Ni hyperaccumulator and a Ni accumulator population) showed that Stenoscepa sp. preferred Ni accumulator Senecio coronatus leaves to all other choices. We conclude that Stenoscepa sp. is extremely Ni‐tolerant. Stenoscepa sp. nymphs prefer leaves of hyperaccumulator Berkheya species, but elevated Ni concentration alone does not determine their food preference. We suggest that the extremely high whole‐body Ni concentration of Stenoscepa nymphs may affect food web relationships in these serpentine communities.

Host plant selection of Chrysolina clathrata （Coleoptera： Chrysomelidae） from Mpumalanga, South Africa

Hyperaccumulated elements such as Ni may defend plants against some natural enemies whereas other enemies may circumvent this defense. The Ni hyperaccumulator Berkheya coddii Roessler (Asteraceae) is a host plant species for Chrysolina clathrata (Clark), which suffers no apparent harm by consuming its leaf tissue. Beetle specimens collected from B. coddii had a whole body Ni concentration of 260 μg/g dry weight, despite consuming leaf material containing 15 100 μg Ni/g. Two experiments were conducted with adults of this beetle species: a no‐choice experiment and a choice experiment. In the no‐choice experiment we offered beetles foliage of one of four species of Berkheya: B. coddii, B. rehmannii Thell. var. rogersiana Thell., B. echinacea (Harv.) O. Hoffm. ex Burtt Davey, and B. insignis (Harv.) Thell. The two former species are Ni hyperaccumulators (defined as having leaf Ni concentration > 1 000 μg/g) whereas the latter have low Ni levels (< 200 μg/g) in their leaves. Masses of beetles were monitored for 6 days. Choice experiments used growing stem tips from the same Berkheya species, placed into Petri dishes with five Chrysolina beetles in each, and the amount of feeding damage caused on each of the four species was recorded. Beetles in the no‐choice experiment gained mass when offered B. coddii, maintained mass on leaves of the other Ni hyperaccumulator (B. rehmannii var. rogersiana), and lost mass when offered non‐hyperaccumulator leaves. In the choice test, beetles strongly preferred B. coddii to other Berkheya species. We conclude that C. clathrata may be host‐specific on B. coddii.